Conventionally, a motor driven, variable displacement, axial piston, hydraulic pump drives a hydraulic device such as a motor or cylinder to operate some type of machine. During the operation of the machine its power requirements may vary widely depending upon the work it is doing. Consequently, the power output of the hydraulic pump which drives it also may vary extensively. Often the power output of the hydraulic pump will be limited only when the pressure of the working fluid at the output port of the pump exceeds a set maximum. For example, a pressure compensated, axial piston, hydraulic pump commonly utilizes a pressure compensating control device which reduces the displacement of the pump when the pressure of the working fluid at the pump outlet port exceeds the pressure setting of the compensating mechanism. Because this device responds only to a set maximum pressure for working pressure fluid at a pump outlet, and works independently of pump displacement, the power output of the pump may vary widely. Thus, the pressure compensating mechanism does not serve to limit the amount of power a pump may absorb.
In some instances, a hydraulic device may demand more power than the motor or prime mover driving it is capable of delivering. This may occur whether the prime mover is driving a single hydraulic device or multiple hydraulic devices. When the hydraulic system absorbs more power than the prime mover is capable of delivering, the prime mover becomes overloaded. If the prime mover is a gasoline or diesel engine, the device may stall. If the prime mover is an electric motor, the electric motor may experience a premature failure. Consequently, it has been found desirable to limit the amount of input horsepower which a hydraulic device such as a variable displacement, axial piston pump may absorb.
Pump horsepower may be determined by multiplying a constant by the flow rate and the pressure of the working fluid output by the pump. One type of power limited device which limits the horsepower output of a variable displacement, axial piston pump to a constant set power may be seen in U.S. Pat. No. 5,183,393 to Schaffner. This device looks at the flow rate and the pressure of the working pressure fluid in the pump outlet. As flow rate changes the pressure setting of a compensator mechanism adjusts to maintain a constant power setting.
It has been found desirable for some applications to provide an easily adjustable displacement control which may be set manually, hydraulically or electro hydraulically from a remote location. The torque limiter device of the instant invention may be adapted easily to act as such a displacement control.